Conventionally, for lighting city streets, natrium lamps, mercury lamps, or metal halide lamps have been generally used. Such lamps require large electric consumption and have a short life. Also, they require high maintenance and repair costs and may cause environmental contamination. Therefore, it can be called a revolutionary change in the lighting field to replace the conventional lighting devices with LED lighting devices.
In general, in an outdoor lamp such as an LED road lamp, light is collected through a lens installed on an LED lamp cover. However, a light field obtained by projecting light through the conventional circular lens has a circular shape, and there is a big difference in brightness between the central part of the light field and the peripheral part thereof. Also, in real application, there are many cases where an illumination range has to be broad in a particular direction and has to be small in another direction.
For example, in a case of a street lighting where a road expands, it is desirable that an illumination range has to be broad along the road with a good lighting effect, and a lighting angle has to be wide in concert with the road extension. However, in the primary direction of the road, an illumination range is preferably small so as to prevent a waste of electricity and pollution of surroundings due to unwanted light. In a case of the conventional LED road lamps equipped with the circular lens, it is generally known that more road lamps must be installed where the road expands so as to secure sufficient lighting effect.
However, in that case, light fields are overlapped with each other so that brightness is higher, but the distribution of lighting intensity is not uniform. Therefore, the resultant light field formed on the road gives a stripe where bright and dark areas are repeatedly formed in turn so that the shape of the light field resembles the ribs. Due to such a lighting phenomenon, the area under lighting devices has high brightness, but other areas have low brightness so that the average brightness of the road is low and is not uniform. As a result, there is much probability that a driver sees an illusion while driving his/her car which can lead to a traffic accident, and also seriously influences safety in driving a vehicle.
Also, besides traffic roads, there is a problem that surroundings are polluted due to stray lights, and glare occurs in various outdoor/indoor environments.